Shrink volume even with ALLOCATE flag

Calling fallocate on the new (smaller) capacity ensures
that the whole file is allocated, but it does not reduce
the file size.

Also call ftruncate after fallocate.

https://bugzilla.redhat.com/show_bug.cgi?id=1366446

src/util/virstoragefile.c

@@ -1338,12 +1338,12 @@ virStorageFileResize(const char *path,
             }
             goto cleanup;
         }
-    } else {
-        if (ftruncate(fd, capacity) < 0) {
-            virReportSystemError(errno,
-                                 _("Failed to truncate file '%s'"), path);
-            goto cleanup;
-        }
+    }
+
+    if (ftruncate(fd, capacity) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to truncate file '%s'"), path);
+        goto cleanup;
     }
 
     if (VIR_CLOSE(fd) < 0) {